1. Field of the Invention
This invention relates to a screw and relates particularly to a recessed head screw capable of engaging a driving tool firmly.
2. Description of the Related Art
Referring to FIG. 1, a conventional screw 1 includes a shank 11, a head 12 disposed at one end of the shank 11, a drilling portion 13 disposed at the other end of the shank 11 and a plurality of threads 14 spirally disposed on the shank 11. The head 12 has a driving socket 121 formed thereon. The driving socket 121, shown in FIG. 2, can be a slotted socket, a cruciform socket, a Pozi socket, a square socket, a hexagonal socket, a star-shaped socket, a Torx socket or other sockets. A driving tool (not shown) engages the driving socket 121 for rotating the screw 1 and drilling the threads 14 into an object (not shown).
For the sake of easy insertion of the driving tool, a dimension of a lower part of the driving tool is slightly less than a bore diameter of the driving socket 121. Therefore, the driving tool and the driving socket 121 are in loose engagement, namely the driving tool does not engage the driving socket 121 closely. However, such structure renders the driving socket 121 unable to receive a full driving force from the driving tool, so the screw 1 cannot be completely driven. Further, when the driving tool works, the driving tool may escape from the driving socket 121 even though the screw 1 does not drop down from the tool at the time of operating the driving tool whose drilling portion 13 is set above the object to execute a downward operation which exerts a downward driving force on the head 12. If the driving tool is set below the object to provide an upward operation which gives the head 12 an upward driving force, the loose engagement between the driving tool and the driving socket 121 causes the screw 1 to fall off the driving tool easily and renders the operation unable to work smoothly.
If the driving socket 121 is a slotted socket, a cruciform socket or a Pozi socket, the driving socket 121 usually has slanting walls. The driving tool may slip out of the driving socket 121 easily because the driving tool is subjected to the loose engagement and the slanting walls. Thus, the driving tool cannot deliver a driving force to the screw 1 efficiently. It is also noted that the user has to hold the driving tool with one hand and hold the screw 1 with the other hand in order to drive the screw 1, regardless of the direction of the driving force. This action generally brings about difficulty in using the driving tool.
To solve the aforementioned problems, a conventional driving tool having the magnetic force has been invented to allow the driving tool to attract the head 12 of the screw 1 for engagement. However, there is a large amount of iron debris during the process of manufacturing large numbers of screw products, so some iron debris may be left within the driving socket 121. However, if the driving tool attracts the iron debris left in the driving socket 121 in the long term, the magnetic force of the driving tool may be reduced, and the reduced magnetic force may also render the driving socket 121 of the screw 1 unable to be driven by the driving tool efficiently. Thus, the working efficiency is still reduced.